1. Field of Invention
This invention relates in general to electrophotographic imaging systems and, more specifically to imaging systems wherein component parts thereof include spherical silicone additives to reduce photoreceptor drag and wear.
2. Description of Related Art
Conventional electrophotographic and electrostatographic imaging systems include photosensitive members, i.e., photoreceptors, that are commonly used in electrophotographic or electrostatographic (Xerographic) processes in either a flexible belt or a rigid drum configuration. The flexible belt can be either seamless or seamed. Photoreceptors generally comprise a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer acts as an insulator in the dark so that electric charges are retained on its surface. When exposed to light the charge dissipates.
A latent image is generally formed on the photoreceptor by first uniformly depositing an electric charge over the surface of the photoconductive layer using one of several methods well known in the art. The photoconductive layer acts as a charge storage capacitor with a charge on its free surface and an equal charge of opposite polarity (the counter charge) on the conductive substrate. A light image is then projected onto the photoconductive layer. On the portions of the photoconductive layer that are exposed to light, the electric charge conducts through the layer and reduces the surface charge. The portions of the surface of the photoconductor that are not exposed to light retain their surface charge. The quantity of electric charge at any particular point on the photoconductive surface is inversely related to the illumination incident thereon, thus forming an electrostatic latent image.
The photodischarge of the photoconductive layer requires that the layer photogenerate a conductive charge and generate and transport this charge through the layer thereby neutralizing the charge on the surface. Two types of photoreceptor structures have been commonly used: multilayer structures wherein separate layers perform the functions of charge generation and charge transport, respectively, and single layer photoconductors that perform both functions. These layers are formed on an electrically conductive substrate and may include an optional charge blocking adhesive layer between the conductive layer and the photoconducting layer or layers. Additionally, the substrate may comprise a non-conducting mechanical support with a conductive surface. Other layers for providing special functions such as incoherent reflection of laser light, dot patterns for pictorial imaging or subbing layers to provide chemical sealing and/or a smooth coating surface may be optionally employed.
As more advanced, higher speed electrophotographic copiers, duplicators and printers have been developed, degradation of image quality has been encountered during extended cycling. During extended cycling, a photoreceptor""s imaging properties degrade as a result of the photoreceptor""s continuous exposure to abrasion, chemical attack, heat and light. Degradation due to cycling is particularly common among multilayered organic photoconductors that use organic film forming polymers and small molecule, low ionization donor material in the charge transport layers. Such wear is further accelerated when the photoreceptor is used in systems that employ abrasive development systems such as single component development systems. Wear is an even greater problem where a drum is used that has such a small diameter that it must rotate many times merely to form a single image for conventional 8.5 inch by 11 inch sized documents. Wear of the photoreceptor can be compensated for by increasing the thickness of the charge transport layer. However, a large increase in charge transport layer thickness can make a photoreceptor inoperable at high imaging process speeds because of the very long transit times of common charge transport layer materials. Also, large decreases in the thickness due to wear can cause dramatic changes in electrical characteristics in only a few thousand cycles that cannot be readily compensated for even with sophisticated computerized control apparatus.
In many of today""s high volume, high precision electrophotographic imaging systems, extended cycling causes adherent spots to form on the photoreceptor""s surface. To prevent imaging defects that can occur as a result of these adherent spots, many modem electrophotographic systems include a spots blade. The function of a spots blade is to remove adherent spots from the surface of the photoreceptor. Conventional cleaning blades work in a similar manner; however, the primary function of a cleaning blade is to remove toner from the photoreceptor""s surfaces.
Spots blades are generally oriented either in a doctor mode or a wiper mode. Unfortunately, because intimate contact between the spots blade and the photoreceptor""s surface is necessary in order to remove the adherent spots, the spots blade often imparts drag to the system that results in higher torque requirements for driving the photoreceptor.
In addition to the frictional wear encountered at the photoreceptor""s outermost layer, another problem encountered when one or more photoconductive layers are applied to a flexible supporting substrate is curling of the photoconductive member. To counteract the curling tendency, imaging engineers have applied coatings to the side of the supporting substrate opposite the photoconductive layers. These coatings, or layers formed by the coatings, are generally referred to as anti-curl layers, backing layers, or anti-curl backing layers. However, difficulties with these anti-curl layers have been encountered. For example, photoreceptor curl can occur in as few as 1,500 imaging cycles under the stressful conditions of high temperature and humidity. Moreover, engineers have found that during the cycling of the photoconductive member, relatively rapid wear of the anti-curl coating causes the photoconductive imaging member to curl. In some tests the anti-curl coating became completely removed in 150,000 to 200,000 cycles. This wear problem becomes even more pronounced when photoconductive members in the form of belts or webs are supported in part by backer bars or stationary guide surfaces that cause the anti-curl layer to wear away very rapidly, producing debris that scatters and deposits on critical machine components, such as lenses, corona charging devices and the like, thereby adversely affecting machine performance.
Also, the anti-curl coatings occasionally separate from the substrate during extended cycling and render the photoconductive imaging member unacceptable for forming quality images. It has also been found that when long webs of a flexible photoconductor having an anti-curl coating on one side of a supporting substrate and a photoconductive layer on the opposite side of the substrate are rolled into large rolls, dimples and creases form on the photoconductive layer that result in print defects in the final developed images. Furthermore, when the webs are formed into belts, segments of the outer surface of the anti-curl layer in contact with each other during shipping or storage at elevated temperatures also cause creases and dimples to form that are seen as undesirable aberrations in the final printed images. Expensive and elaborate packaging is necessary to prevent the anti-curl coating from contacting itself. Additional difficulties have been encountered in continuous coating machines during the winter manufacturing of coated photoconductive imaging members because of occasional seizing that prevents transport of the coated web through the machine for downstream coating.
Numerous attempts have been made to reduce frictional damage to electrophotographic imaging system photoreceptors. Unfortunately, each solution often leads to additional problems. Moreover, since most of the solutions are aimed at reducing friction between the photoreceptor""s outermost surface and the imaging system""s cleaning blade, many of these solutions do not provide a way to reduce frictional damage caused by interaction with a system""s spots blade or frictional damage occurring at the photoreceptor""s backside anti-curl layer.
For instance, one solution includes adding a lubricant such as wax to the toner. However, the problem with this approach is that the fixability of the toner can degrade the toner""s electrical function, or further filming can occur causing poor image quality.
In other attempts to reduce frictional damage, efforts have been made to reduce frictional forces by applying a lubricant to the outer surface of the photoreceptor. For instance, U.S. Pat. No. 5,721,085 to Oshiba et al. discloses a method for incorporating a lubricating substance on a photoreceptor""s outermost surface. Although this approach is effective in reducing frictional forces encountered at the photoreceptor""s outermost surface, the solution does not address frictional forces encountered at the photoreceptor""s anti-curl layer.
Others have attempted to reduce photoreceptor drag and wear by adding a lubricant to an eletrophotographic imaging system""s cleaning blade. U.S. Pat. No. 4,519,698 to Kohyama et al., for instance, discloses a method in which a waxy lubricant is used to continuously lubricate a cleaning blade. However, the problem with this solution is that the thickness of the lubricant film that forms on the photosensitive drum is difficult to maintain, and interference with the electrostatic characteristics of the photosensitive member occurs. U.S. Pat. No. 5,819,147 to Shoji, discloses an image forming apparatus that uses a silicone resin lubricant deposited at a contact region between an image bearing member and a cleaning blade. Again, while this solution effectively reduces frictional forces between the cleaning blade and the photoreceptor""s outer surface, and might also be useful in reducing friction between the photoreceptor and a spots blade, it does not address frictional damage to the photoreceptor""s anti-curl layer.
Attempts have also been made to reduce photoreceptor wear by constructing cleaning blades out of materials having a low coefficient of friction. However, these attempts have been plagued by the degradation of other important system characteristics, especially mechanical strength, due to the presence of additives.
According to U.S. Pat. No. 4,340,658 to Inoue et al. and U.S. Pat. No. 4,388,392 to Kato et al., surface smoothness of a photosensitive layer can be improved by adding a leveling agent such as polydimethylsiloxane to a polyvinyl carbazole type photoconductor.
In addition, when attempts have been made to reduce friction by spraying conventional silicone oil onto the imaging surface of a charge transport layer, it has been discovered that the charge transport layer cracked when bent, even without being cycled.
Photoreceptors containing trace quantities of silicone oil are also generally known in the art. Engineers have used trace quantities of silicone oil to improve surface properties of the dried film and to improve out flow during the coating process. The quantity of silicone oil used in these processes is too small to affect the bulk properties of the dried film. However, because silicone oil is quickly removed during normal abrasive wear due to toner, cleaning blades, brushes, paper contact and other frictional forces encountered during processing, any lubricating effect of silicone oil on a photoreceptor""s surface is short lived.
Thus, it is desirable to increase the durability and to extend the life of the exposed surfaces of a photoreceptor, as well as to reduce the frictional contact between the components of an electrophotographic imaging system while maintaining electrical and mechanical integrity and print quality.
The present invention is directed to use of silicone additives, particularly a spherical silicone additive, to reduce photoreceptor drag and wear. More specifically, the invention is directed to introducing such an additive into a contact region between a backside anti-curl layer of the photoreceptor and an electrophotographic imaging system""s backer bar or other components, and/or contact regions between an outermost layer of the photoreceptor and an electrophotographic imaging system""s spots blade.
First, to reduce photoreceptor drag and wear at a contact region between a photoreceptor""s anti-curl layer and an electrophotographic system""s backer bar, a small amount of silicone additive can be applied to a photoreceptor""s anti-curl layer surface and/or backer bar surface. Because of the lubricating, non-migratory nature of specific forms of silicone additives, such additives can impart lubricity and wear properties to a material similar to that achieved by conventional migratory slip additives without the risk of severe filming in very small concentrations. The enhanced lubricity and wear resistance associated with these additives can reduce photoreceptor drag and extend photoreceptor life.
In addition to reducing drag and wear, other potential benefits of introducing a silicone additive to the surface of the photoreceptor include: decreased photoreceptor belt drag, reduction of the possibility of chatter due to friction, and an increase in the allowable blade angle before chatter and/or blade tuck occurs.
The present invention is similarly directed to reducing frictional forces at a contact region between a system""s spots blade and the outermost surface of the photoreceptor. In an effort to reduce the drag caused by friction between the spots blade and the photoreceptor""s surface, a silicone additive can be added to a bulk material or surface layer material of the blade during fabrication.
The introduction of silicone additives to the contact regions between photoreceptor""s backside anti-curl layer and the system""s backer bar and the photoreceptor""s outermost layer and the system""s spots blade can reduce photoreceptor irregularities, excessive backer bar wear, excessive spots blade wear, belt ripple and belt contamination. Moreover, by reducing the amount of drag and wear occurring at these contact regions, the present invention can also help to minimize torque requirements for driving the photoreceptor.